Apparatus for measuring small changes in liquid level in a container

ABSTRACT

A liquid level sensing apparatus for detecting leaks in storage tanks. A buoyancy member is at least partially immersed in the liquid, suspended from a pivoted, counterbalanced, parallelogram support arrangement with a radio frequency target sensing element for electrical readout of displacement. Two such apparatus are incorporated in a system with a provision for measuring a portion of the liquid of the tank in isolation from the remainder, in order to provide for temperature compensation.

This is a continuation of application Ser. No. 646,758, filed Sept. 4,1984, and now abandoned.

BACKGROUND OF THE INVENTION

In the testing of underground tanks such as gasoline tanks for leaks, ithas heretofore been considered essential that the temperature of thecontents of the tank be monitored during the period of the test. Thecoefficient of expansion of gasoline is such that a change intemperature of 1° F. of the contents of a 5000 gallon tank will cause achange in volume of about 3.25 gallons. Since the specifications of theNational Fire Protection Association require that the rate of leakingfrom such a tank should not exceed 0.05 gallons per hour, it can be seenthat if the test requires an hour or more to complete, the change intemperature during the test can give an erroneous result if this changeis not taken into account. If the contents of the tank are decreasing intemperature, an erroneous indication of tank leaking may be given,whereas if the contents of the tank are increasing in temperature, thepresence of an actual leak may be disguised by the increase in volume ofthe tank contents due to the temperature increase.

Another problem relating to temperature change is the fact that over aperiod of only an hour, the rate at which the temperature of the tankcontents changes can vary substantially.

Various means for detecting such leaks and for compensating for thechange in volume of the tank contents due to temperature change andevaporation during the test have been proposed. For example, U.S. Pat.No. 4,186,591 issued Feb. 5, 1980 shows a tank testing system havingmeans for simultaneously monitoring the temperature and the level of thetank liquid. U.S. Pat. No. 4,300,388 issued Nov. 17, 1981 shows a tanktesting device which utilizes a buoyant member suspended in the tankliquid, with means for compensating for changes in liquid level due tovaporization of the liquid and temperature changes. Obviously suchcompensation is necessary only if the test must be carried out over aconsiderable period of time. The above mentioned U.S. Pat. No. 4,300,388utilizes a laboratory balance for supporting the buoyancy member todetect the change in level of the liquid, however it has been found thatordinary chemical or laboratory balances are not entirely satisfactoryfor such tank testing for a number of reasons.

In the case of a tank having a leak of the minimum amount required to bedetected, (0.05 gallons per hour) the rate of change of the liquid levelin the 4" fill pipe of the tank is only about 0.017 inches per minute.Although such scales are capable of accurately measuring small weightdifferences (which often requires that the beam be physically moved tothe lower limit of its travel and then released to insure that the beamcomes accurately to the balanced position), such scales are not suitablefor measuring small increments of movement of a device suspended fromthe scale beam, since the scale is supported on a so-called knife edge.However such "knife-edge" does not actually provide an infinitely thinline on which the beam rests, but actually provides a narrow elongatedsurface having a finite width on which the beam pivots. This surface isalso not absolutely regular, and therefore will cause a slightresistance to the starting motion of the beam, particularly when a verysmall force is applied very slowly and gradually to the beam. When theforce reaches a sufficient amount, the beam will move suddenly, and willmove slightly further than a distance which is proportional to theapplied force.

This effect is sometimes referred to in the industry as "stiction", andhas little consequence when the scale is used for measuring weight,since the beam can, if necessary, be moved by hand to a stop asdescribed above, so that the force tending to restore the scales to thebalanced position is relatively large. However, when the scale is usedfor detecting small changes in liquid level that occur over a period oftime, with the beam position being accurately detected electronically,and with a chart recorder read-out of beam position, this erratic motioncan cause wide fluctuations of the chart recorder pen which areconsiderably greater than the change due to liquid level change alone,and therefore require an extended test period to determine the "line ofbest fit " on the chart for actual rate of change of the liquid level.

SUMMARY OF THE INVENTION

This invention provides apparatus and method for measuring the change inlevel of liquid in a container, and is particularly adapted for use indetecting leaks in underground storage tanks.

The apparatus comprises a parallelogram structure mounted on a fixedbase, with a stationary end and a movable end which is capable ofvertical movement which is absolutely proportional to the force applied.The movable end has a support arm for a buoyancy device to be suspendedso as to be partially submerged in the liquid, and also carries an RFtarget plate. A radio frequency generator is mounted on the fixed baseand projects a low level RF signal, which generates eddy currents in thetarget plate, which reduces the impedance of the sensor. This change inimpedance is directly proportional to the distance between the targetplate and the RF generator, and can provide a voltage output, the changein which can be measured to determine the change in position of thetarget plate.

The parallelogram pivots are not rotating bearings, but are flexureconnectors joining the parallelogram members so that no surfaces of themembers are in contact with each other. The motion of the parallelogramis therefore essentially frictionless and is accurately proportional tothe force applied through the limited distance the support arm must moveto accomodate changes in liquid level, even at the extremely slow rateof change of liquid level when a small leak is present. Suitable dampingmeans is also provided between the parallelogram and its support toreduce the effect of ground vibrations and the effect of wind across thefill pipe of the tank being tested. In a preferred embodiment of theinvention, the scales are mounted onto a support which has a suitablefitting to enable it to be mounted directly onto the fill pipe of thetank, to prevent vibrations of the surrounding ground from reaching thescales.

It has been found that the apparatus described herein is easily capableof accurately providing a uniform output signal during extremely lowrates of change of liquid level, and that an accurate indication oflevel change can be obtained in a test period short enough that in mostcases, any change in liquid level due to temperature change orevaporation during the period of the test can be ignored. The apparatusis also accurate and sensitive enough to detect a change in the rate ofleaking in situations where the leak is in the piping above the tank.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 is a perspective view of a liquid level testing apparatusembodying the features of the invention.

FIG. 2 is a view in section, partly in elevation, illustrating the leaktester of FIG. 1 mounted onto the fill pipe of an underground tank inposition for performing a leak test on the tank.

FIG. 3 is an enlarged view of the mounting assembly by which the testingapparatus is mounted onto the fill pipe.

FIG. 4 is an enlarged perspective view of one of the pivots of theapparatus of FIG. 1.

FIG. 5 is an exploded view of the flexible pivot member of FIG. 4

FIG. 6 is a graph illustrating the drop of liquid level in the tank fillpipe over a period of time as measured by the output of the apparatus ofFIG. 1 applied to a chart recorder.

FIG. 7 is another graph illustrating the change in rate of leakage of atank in which the leak is in the piping above the tank.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring to the drawing, there is illustrated a liquid level testingapparatus 10 which is particularly adapted for detecting small changesin liquid level in a storage tank.

The illustrated embodiment of the apparatus comprises a support plate 12on which is mounted a beam support member 14 which extends verticallyfrom the support plate 12 and supports a movable beam which comprisesupper and lower beam members 16 and 18. Each beam support member hasrearwardly extending legs 20 and 22 which are spaced apart a suitabledistance to enable them to straddle the beam support member 14 and areattached thereto at pivot points 24 in a manner to be described.

The forward ends 26 and 28 of the upper and lower beam members 16 and 18are joined by a vertically extending member 30, which is parallel to thebeam support member 14, at pivot points 32.

Extending forwardly from the connecting member 30 is a support arm 34for supporting a buoyancy member 36.

The beam support member 14, the beam members 16 and 18, and theconnecting member 30 form a parallelogram, so that movement of the beammembers 16 and 18 about the pivot points causes vertical motion of thesupport arm 34. The amount of vertical motion of the buoyancy member 36is therefore independent of the position on the arm 34 to which thebuoyancy member is attached.

To balance the beam to a desired starting position, and to compensatefor the weight of the buoyancy member 36, a pair of counter-weights 38are mounted on threaded members 40 extending rearwardly from the upperbeam member 16.

To damp out oscillations of the beam, a self-contained damping device 42is mounted on the beam support member 14, and has an operating rod 44extending therefrom with the remote end thereof projecting into asuitable aperture in the upper beam member 16, so that vertical motionof the beam causes pivoting of the operating rod.

The apparatus 10 is primarily intended for detecting small changes inthe level of liquid in a storage tank by detecting the change inbuoyancy of the member 36 partially suspended in the liquid by measuringthe change in position of the beam over a period of time. If a leakexists in the tank, the liquid level will drop, so that the buoyancymember will also tend to drop, moving the support arm downwardly.

As stated hereinbefore, such movement is at an extremely slow rate, andtherefore the pivot points must be essentially frictionless. Thereforethe mounting of the upper and lower beam members to the beam supportmember, at the pivot points 24, and the mounting of the connectingmember 30 to the forward ends of the beam members at pivot points 32must be essentially frictionless.

For this purpose the members comprising the parallelogram are spacedlaterally from each other at the pivot points and are joined byso-called flexural pivots (see FIGS. 4 and 5) which comprise twoco-axial end portions 46 and 48 which are press fitted into suitableapertures in adjacent spaced surfaces of the members (such as theforward end 28 of the lower beam member and the lower end of theconnecting member 30) so that one end portion is fixedly retained ineach member.

Each end portion comprises a retaining portion (50 and 52) which iscurved to conform to the inner surface of the end portion and isresiliently retained therein, and a flexible tongue (54 and 56) whichextends into the interior of the other end portion and straddles therear end of the other tongue inside the retaining portion via respectiveslots 55, 57.

The portions 46, 48 are therefore joined only by said flexible tongues54, 56 and are at no point in sliding, rolling, or pivoting contact, asis the case with the ordinary chemical or laboratory balance. Rotationof one end portion in relation to the other end portion causes flexingof said tongues and within the design limits of the flexural members,the angular displacement is accurately proportional to the force appliedby the buoyancy member, even at extremely low forces.

A force applied to the support arm 34 is therefore resisted by theflexible members 54, 60 and the motion of the arm and the parallelogramis accurately proportional to the force applied to the support arm.

To measure the movement of the support arm from a predetermined positionwith the required accuracy, an electronic non-contact measuring deviceis provided. In the illustrated embodiment this comprises a low-level RFfield generator 66 mounted on the support 12, and a target plate 68mounted on the movable end of the parallelogram. The RF generator 66generates a low level RF field that generates eddy currents in thetarget 68, which eddy currents reduce the impedance of the sensor. Thisreduction in impedance is utilized by suitable electronic circuitry toproduce a voltage that is linearly proportional to the distance of thesensor face to the surface of the target. Such devices are commerciallyavailable, and are capable of providing a 200 millivolt change involtage with a change in distance to the target plate of .001 inch.

The output of the sensor may be fed to a chart recorder to provide acontinuous read-out of the motion of the target plate, (corresponding tothe change in level of the liquid in the storage tank,) and to therebyprovide, over a period of time, a graph showing the change in level ofthe liquid in the tank.

The initial portion of each graph of FIGS. 6 and 7 shows oscillationsdue to the disturbance in the tank caused by the insertion of the tube Tand the buoyancy member 36. After this initial oscillation has subsided,a measured quantity of liquid previously taken from the tank (in thiscase, 50 ml.) is added to the fill pipe to calibrate the recorder, whichresults in a rise in level of the liquid in the tube corresponding to 12divisions on the chart. The relationship between change of volume in thefill pipe and any other piping above the surface of the tank and thechart divisions is therefore established for this particular tankinstallation.

As the test progresses, it is seen that oscillations occur due to groundvibrations and the effect of wind on the level of fluid in the tank fillpipe, however, within 15 minutes, a line of best fit (L) can beestablished, indicating a rate of drop of level in the fill pipe ofabout 0.12 inch in 15 minutes, indicating a leak of about 90 cc per 15minutes, equivalent to about 0.024 gallons per hour, or less than halfof the amount of leakage permitted by the NFPA specification.

If desired, the change in volume of the liquid in the tank due totemperature change may be monitored by providing a tube T full of liquidfrom the tank. The tube T has an open top end and extends fromsubstantially the bottom of the tank into the fill pipe (see FIG. 2). Asecond scales 10T may be provided, which may be identical to scales 10,with a buoyancy member 36T suspended from the support arm 34T into theliquid in the tube T, to detect changes in the level of liquid in thetube T, which will be caused solely by the change in temperature of thetank contents.

If the rate of change of temperature of the tank contents were constant,the same results should be obtained regardless of the duration of thetest. However, it has been found that if the temperature is monitoredover a period of an hour or more, it is found that the rate of change oflevel in the temperature test pipe can vary considerably in eitherdirection. Therefore a test method that requires monitoring the tanklevel for an extended period of time, and measuring the difference infill pipe level that occurs over that period of time can be considerablyin error due to changes in the rate of temperature change during thetest.

It has been found that the apparatus described herein is easily capableof an accurate indication of a change in liquid level caused by a leakof 0.05 gallons per hour (11.55 cubic inches, or about 190 cc per hour)which is the maximum rate of leakage permitted under the NFPAregulations.

This rate of leakage causes a drop in fill pipe level of about 0.25 inchin 15 minutes. However the equipment described herein is easily capableof detecting even smaller rates of change of liquid level, and can,without difficulty a detect a change in liquid level of as little as0.025 inch per 15 minutes, equivalent to a leak of only 0.0065 gallonsper hour, or 4.7 cc per 15 minutes.

Therefore by providing an apparatus that give an accurate test of liquidlevel in the fill pipe in 15 minutes or less, any effects due totemperature change or evaporation can be ignored.

However, if desired, such as in situations where regulations requirethat the change in volume due to temperature change during the test bemonitored and recorded, the output of the second scale 10T may also beapplied to the chart recorder, and the percentage change of the liquidin the tube T applied to the total volume in the tank to obtain thechange in fill pipe level due to temperature alone. However in mostcases it has been found that the rate of change of tank temperature isso slow that it is unnecessary to take into account the temperaturechange of the tank contents during the short period of the test.

It has been found that the accuracy and sensivity of the apparatusenables it, in some cases, to indicate the position of the leak. Forexample, FIG. 7 is a graph of a leak in which the rate of leakingdecreases as the test proceeds. (The four sections, A, B, C, and D ofthe graph are continuations of the same test, since when the chart penreaches the left edge of the chart paper, it is reset by electricalbiasing to the right edge of paper). Portion B of the graph has a slopein relation to vertical, or zero slope which is less than the slope ofthe previous portion A, and portion C has a slope less than that of B,with portion D having a slope approaching zero. It has been found thatthis phenomena occurs when the leak is at some point above the tankitself, such as at the junction of the fill pipe with the tank or in thepiping to the pump or to the vent pipe. The rate of leaking decreases asthe level of the liquid in these pipes approaches the position of theleak, thereby decreasing the head of fluid and reducing the rate of theleak.

This ability of the apparatus to detect the fact that the leak is in thepiping or connections above the tank is especially valuable, since iteliminates the need of digging up the entire tank to find the leak.

In a preferred embodiment of the invention, the scales 10 and 10T may bemounted in a suitable enclosure 70, which has a support column 72 on thebottom with a fitting on the bottom end thereof which is similar to thefitting on the end of a fill hose, so that the enclosure may be mounteddirectly onto the fill pipe, without contact of the enclosure with thesurrounding surface (usually concrete or asphalt) so that the scales areisolated from receiving vibrations directly from the surrounding surfaceof the earth. A suitable aperture 74 is provided in the bottom of theenclosure for allowing the bouyancy members and supporting line to hangfrom the support arms of the scales into the fill pipe. The enclosurealso assists in reducing the effect of wind on the apparatus.

Since certain changes apparent to one skilled in the art may be made inthe herein described embodiments of the invention without departing fromthe scope thereof, it is intended that all matter contained herein beinterpreted in an illustrative and not a limiting sense.

We claim:
 1. Apparatus for determining small changes in the level ofliquid in a container comprising:a frame including a rigidly mounted,vertical support member; a pivoting mechanism including a pair ofhorizontal members pivotably joined to a pair of vertical members in agenerally parallelogram configuration, the vertical support member ofthe frame constituting a first one of the pair of vertical members ofthe pivoting mechanism; at least one of said horizontal memberscomprising a Y-shaped yoke pivotably mounted at two points to a pair ofspaced apart support elements of the vertical support member; a buoyancymember; vertically movable means for supporting the buoyancy member inat least partial immersion in the liquid within the container, saidsupporting means being vertically movable in response to minute changesof buoyant force on the buoyancy member resulting from minute changes inliquid level; means for balancing the buoyancy member at a selectedposition in preparation for taking measurements of liquid level todetermine changes in excess of a specified rate; and flexing meanscoupling one of the horizontal members to the vertical support member ofthe frame for damping oscillations of the pivoting mechanism.
 2. Theapparatus of claim 1 wherein the at least one horizontal membercomprising a Y-shaped yoke is positioned above the other horizontalmember of said pair.
 3. The apparatus of claim 2 wherein the Y-shapedyoke comprises a pair of rearwardly extending legs which straddle thespaced apart support elements of the vertical support member and extendpast the vertical support member.
 4. The apparatus of claim 3 whereinsaid rearwardly extending legs include threaded extended portions andwherein the balancing means include a pair of weights, each of which isthreaded onto a corresponding threaded extended portion, the weightsbeing movable along the threaded portions by rotation thereon to balancethe buoyancy member at said selected position.
 5. The apparatus of claim3 wherein the remaining horizontal member of said pair also comprises aY-shaped yoke pivotably mounted at two points to said pair of spacedapart support elements, the pivotable mounting points of the lowerhorizontal member being vertically aligned along the spaced apartsupport elements with corresponding ones of the pivotable mountingpoints for the upper horizontal member.
 6. The apparatus of claim 5wherein the Y-shaped yoke of the lower horizontal member comprises apair of rearwardly extending legs which straddle the spaced apartsupport elements of the vertical support member and extend past thevertical support member.
 7. The apparatus of claim 6 wherein theremaining vertical member of the pivoting mechanism comprises a singlebar pivotably mounted at opposite ends thereof to corresponding ends ofthe horizontal members which are remote from the pivotable mounting tothe spaced apart support elements of the vertical support member of theframe.
 8. The apparatus of claim 7 wherein said frame further includes ahorizontal plate rigidly attached to said rigidly mounted verticalsupport member, said frame having a longitudinal axis extendinggenerally centrally thereof and orthogonal to the vertical supportmember of the frame.
 9. The apparatus of claim 8 wherein the verticallymovable means include an arm extending orthogonally to the verticalsupport bar and generally outward therefrom along the longitudinal axisof the frame.
 10. The apparatus of claim 8 wherein the frame includes atleast one adjustable levelling member threadably coupled to thehorizontal plate for levelling the frame.
 11. The apparatus of claim 9wherein the vertical movable means include a support line coupledbetween the buoyancy member and said arm for supporting the buoyancymember therefrom.
 12. The apparatus of claim 11 wherein the upper end ofsaid line is attached to an outer end of said arm.
 13. The apparatus ofclaim 9 further including measuring means for sensing vertical movementof the horizontal arm.
 14. The apparatus of claim 13 wherein themeasuring means comprise a reference element affixed to said single barto move therewith and a transducer mounted to the frame adjacent thereference element to sense movement of said reference element.
 15. Theapparatus of claim 14 wherein said transducer comprises an RF fieldgenerator within a transducer housing mounted to the frame, saidtransducer having a flat end surface adjacent said reference element.16. The apparatus of claim 1 further including a plurality of resilientpivot members for pivotably mounting the horizontal and vertical membersof the parallelogram configuration, said resilient members comprising apair of mating resilient elements, each resilient element having acurved portion for fitting within a round mounting hole and a centrallypositioned flat extending portion slotted to receive the correspondingportion of the other resilient element of said pair to permit limitedrotational movement of one member of the pivoted mechanism relative tothe other member of the pivoted mechanism having a common resilientcoupling therewith by virtue of the flexibility of said flat portions.17. Apparatus for determining small changes in the level of liquid in acontainer comprising:a frame including a rigidly mounted, verticalsupport member; a pivoting mechanism including a pair of horizontalmembers pivotably joined to a pair of vertical members in a generallyparallelogram configuration, the vertical support member of the frameconstituting a first one of the pair of vertical members of the pivotingmechanism; at least one of said horizontal members comprising a Y-shapedyoke pivotably mounted at two points to a pair of spaced apart supportelements of the vertical support member; a buoyancy member; verticallymovable means for supporting the buoyancy member in at least partialimmersion in the liquid within the container, said supporting meansbeing vertically movable in response to minute changes of buoyant forceon the buoyancy member resulting from minute changes in liquid level;means for balancing the buoyancy member at a selected position inpreparation for taking measurements of liquid level to determine changesin excess of a specified rate; and a damping device mounted on thevertical support member in line with said at least one horizontal membercomprising a Y-shaped yoke at a location generally centrally locatedbetween said spaced apart support elements and having an elongatedmember extending to be coupled with said at least one horizontal memberto damp out oscillations of the pivoting mechanism.
 18. The apparatus ofclaim 17 wherein said damping device further includes a button-shapedfirst end adjacent said vertical support member to provide a resilientmounting to said elongated member.
 19. The apparatus of claim 18 whereinsaid elongated member comprises a thin rod extending from thebutton-shaped first end into an aperture in that surface of the Y-shapedyoke facing toward the vertical support member.